Re: Low loss/dissipation xistor or ...

On Fri, 19 Dec 2008 20:44:12 -0500, Bob Engelhardt wrote:

The project:
I am making electrically heated socks & gloves. For power I'm using the
9.6v NiCad batteries from my cordless drill. The working assumption is
that each sock & glove will need 1w, but that's very tentative & I'll
have a regulator to adjust as needed. The critical parameter is the
9w-hr available from a battery before needing to be recharged. So the
regulator must have minimum losses. The 2 socks & the 2 gloves will be
in series & independently regulated.

The current plan is to use a 555 timer for each pair & vary the duty
cycle to achieve regulation. But the 555 can't drive the heaters
directly - they'll require about an amp. I'm thinking of using SMT
BJT's from ON Semi:
http://www.onsemi.com/PowerSolutions/parametrics.do?id=808 because the
very low VCEsat (as low as 50mv) will minimize the regulator losses.

The questions:
- is there another device that I could use instead of BJT's? Switching
10v & carrying an amp at low loss (1/10w +-). (OK, now it's obvious - I
don't know anything about semiconductors <G>)

- are there through-hole BJT's with very low VCEsat? (I've not used
SMT's & I suspect that they'll be a fabrication challenge.) Googling on
"low VCEsat BJT" was not helpful.

Thanks,
Bob

BTW - I hang around rec.crafts.metalworking mostly & when I subscribed
here I was REALLY impressed the the minimal amount of OT posts. No
political OT that I recall. Keep up the good work.

If you put the transistor in the sock, then any losses from the
transistor will heat the sock just as well as a resistor would.

In fact, if you controlled it right you could just use the transistor in
linear mode, and forget the resistor.

But why not just use a resistor with the right value to dissipate a watt
with a 9V input? You'll see a bit of extra dissipation with fully
charged batteries and less at low, but it won't be noticeable.

(actually, IMHO, you shouldn't sweat the "constant power" issue at all.
What you should concern yourself with is a circuit to shut the thing down
when the batteries get to the end of their charge, to avoid damaging the
pack).

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
.

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